Raw pointers (C++)

Article
11/07/2022

A pointer is a type of variable. It stores the address of an object in memory, and is used to access that object. A raw pointer is a pointer whose lifetime isn't controlled by an encapsulating object, such as a smart pointer. A raw pointer can be assigned the address of another non-pointer variable, or it can be assigned a value of nullptr. A pointer that hasn't been assigned a value contains random data.

A pointer can also be dereferenced to retrieve the value of the object that it points at. The member access operator provides access to an object's members.

    int* p = nullptr; // declare pointer and initialize it
                      // so that it doesn't store a random address
    int i = 5;
    p = &i; // assign pointer to address of object
    int j = *p; // dereference p to retrieve the value at its address

A pointer can point to a typed object or to void. When a program allocates an object on the heap in memory, it receives the address of that object in the form of a pointer. Such pointers are called owning pointers. An owning pointer (or a copy of it) must be used to explicitly free the heap-allocated object when it's no longer needed. Failure to free the memory results in a memory leak, and renders that memory location unavailable to any other program on the machine. Memory allocated using new must be freed by using delete (or delete[]). For more information, see new and delete operators.

    MyClass* mc = new MyClass(); // allocate object on the heap
    mc->print(); // access class member
    delete mc; // delete object (please don't forget!)

A pointer (if it isn't declared as const) can be incremented or decremented to point at another location in memory. This operation is called pointer arithmetic. It's used in C-style programming to iterate over elements in arrays or other data structures. A const pointer can't be made to point to a different memory location, and in that sense is similar to a reference. For more information, see const and volatile pointers.

    // declare a C-style string. Compiler adds terminating '\0'.
    const char* str = "Hello world";

    const int c = 1;
    const int* pconst = &c; // declare a non-const pointer to const int
    const int c2 = 2;
    pconst = &c2;  // OK pconst itself isn't const
    const int* const pconst2 = &c;
    // pconst2 = &c2; // Error! pconst2 is const.

On 64-bit operating systems, a pointer has a size of 64 bits. A system's pointer size determines how much addressable memory it can have. All copies of a pointer point to the same memory location. Pointers (along with references) are used extensively in C++ to pass larger objects to and from functions. It's often more efficient to copy an object's address than to copy the entire object. When defining a function, specify pointer parameters as const unless you intend the function to modify the object. In general, const references are the preferred way to pass objects to functions unless the value of the object can possibly be nullptr.

Pointers to functions enable functions to be passed to other functions. They're used for "callbacks" in C-style programming. Modern C++ uses lambda expressions for this purpose.

Initialization and member access

The following example shows how to declare, initialize, and use a raw pointer. It's initialized using new to point an object allocated on the heap, which you must explicitly delete. The example also shows a few of the dangers associated with raw pointers. (Remember, this example is C-style programming and not modern C++!)

#include <iostream>
#include <string>

class MyClass
{
public:
    int num;
    std::string name;
    void print() { std::cout << name << ":" << num << std::endl; }
};

// Accepts a MyClass pointer
void func_A(MyClass* mc)
{
    // Modify the object that mc points to.
    // All copies of the pointer will point to
    // the same modified object.
    mc->num = 3;
}

// Accepts a MyClass object
void func_B(MyClass mc)
{
    // mc here is a regular object, not a pointer.
    // Use the "." operator to access members.
    // This statement modifies only the local copy of mc.
    mc.num = 21;
    std::cout << "Local copy of mc:";
    mc.print(); // "Erika, 21"
}

int main()
{
    // Use the * operator to declare a pointer type
    // Use new to allocate and initialize memory
    MyClass* pmc = new MyClass{ 108, "Nick" };

    // Prints the memory address. Usually not what you want.
    std:: cout << pmc << std::endl;

    // Copy the pointed-to object by dereferencing the pointer
    // to access the contents of the memory location.
    // mc is a separate object, allocated here on the stack
    MyClass mc = *pmc;

    // Declare a pointer that points to mc using the addressof operator
    MyClass* pcopy = &mc;

    // Use the -> operator to access the object's public members
    pmc->print(); // "Nick, 108"

    // Copy the pointer. Now pmc and pmc2 point to same object!
    MyClass* pmc2 = pmc;

    // Use copied pointer to modify the original object
    pmc2->name = "Erika";
    pmc->print(); // "Erika, 108"
    pmc2->print(); // "Erika, 108"

    // Pass the pointer to a function.
    func_A(pmc);
    pmc->print(); // "Erika, 3"
    pmc2->print(); // "Erika, 3"

    // Dereference the pointer and pass a copy
    // of the pointed-to object to a function
    func_B(*pmc);
    pmc->print(); // "Erika, 3" (original not modified by function)

    delete(pmc); // don't forget to give memory back to operating system!
   // delete(pmc2); //crash! memory location was already deleted
}

Pointer arithmetic and arrays

Pointers and arrays are closely related. When an array is passed by-value to a function, it's passed as a pointer to the first element. The following example demonstrates the following important properties of pointers and arrays:

The sizeof operator returns the total size in bytes of an array
To determine the number of elements, divide total bytes by the size of one element
When an array is passed to a function, it decays to a pointer type
When the sizeof operator is applied to a pointer, it returns the pointer size, for example, 4 bytes on x86 or 8 bytes on x64

#include <iostream>

void func(int arr[], int length)
{
    // returns pointer size. not useful here.
    size_t test = sizeof(arr);

    for(int i = 0; i < length; ++i)
    {
        std::cout << arr[i] << " ";
    }
}

int main()
{
    int i[5]{ 1,2,3,4,5 };
    // sizeof(i) = total bytes
    int j = sizeof(i) / sizeof(i[0]);
    func(i,j);
}

Certain arithmetic operations can be used on non-const pointers to make them point to another memory location. Pointers are incremented and decremented using the ++, +=, -= and -- operators. This technique can be used in arrays and is especially useful in buffers of untyped data. A void* gets incremented by the size of a char (1 byte). A typed pointer gets incremented by size of the type it points to.

The following example demonstrates how pointer arithmetic can be used to access individual pixels in a bitmap on Windows. Note the use of new and delete, and the dereference operator.

#include <Windows.h>
#include <fstream>

using namespace std;

int main()
{
    BITMAPINFOHEADER header;
    header.biHeight = 100; // Multiple of 4 for simplicity.
    header.biWidth = 100;
    header.biBitCount = 24;
    header.biPlanes = 1;
    header.biCompression = BI_RGB;
    header.biSize = sizeof(BITMAPINFOHEADER);

    constexpr int bufferSize = 30000;
    unsigned char* buffer = new unsigned char[bufferSize];

    BITMAPFILEHEADER bf;
    bf.bfType = 0x4D42;
    bf.bfSize = header.biSize + 14 + bufferSize;
    bf.bfReserved1 = 0;
    bf.bfReserved2 = 0;
    bf.bfOffBits = sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER); //54

    // Create a gray square with a 2-pixel wide outline.
    unsigned char* begin = &buffer[0];
    unsigned char* end = &buffer[0] + bufferSize;
    unsigned char* p = begin;
    constexpr int pixelWidth = 3;
    constexpr int borderWidth = 2;

    while (p < end)
    {
            // Is top or bottom edge?
        if ((p < begin + header.biWidth * pixelWidth * borderWidth)
            || (p > end - header.biWidth * pixelWidth * borderWidth)
            // Is left or right edge?
            || (p - begin) % (header.biWidth * pixelWidth) < (borderWidth * pixelWidth)
            || (p - begin) % (header.biWidth * pixelWidth) > ((header.biWidth - borderWidth) * pixelWidth))
        {
            *p = 0x0; // Black
        }
        else
        {
            *p = 0xC3; // Gray
        }
        p++; // Increment one byte sizeof(unsigned char).
    }

    ofstream wf(R"(box.bmp)", ios::out | ios::binary);

    wf.write(reinterpret_cast<char*>(&bf), sizeof(bf));
    wf.write(reinterpret_cast<char*>(&header), sizeof(header));
    wf.write(reinterpret_cast<char*>(begin), bufferSize);

    delete[] buffer; // Return memory to the OS.
    wf.close();
}

`void*` pointers

A pointer to void simply points to a raw memory location. Sometimes it's necessary to use void* pointers, for example when passing between C++ code and C functions.

When a typed pointer is cast to a void pointer, the contents of the memory location are unchanged. However, the type information is lost, so that you can't do increment or decrement operations. A memory location can be cast, for example, from MyClass* to void* and back again to MyClass*. Such operations are inherently error-prone and require great care to avoid errors. Modern C++ discourages the use of void pointers in almost all circumstances.

//func.c
void func(void* data, int length)
{
    char* c = (char*)(data);

    // fill in the buffer with data
    for (int i = 0; i < length; ++i)
    {
        *c = 0x41;
        ++c;
    }
}

// main.cpp
#include <iostream>

extern "C"
{
    void func(void* data, int length);
}

class MyClass
{
public:
    int num;
    std::string name;
    void print() { std::cout << name << ":" << num << std::endl; }
};

int main()
{
    MyClass* mc = new MyClass{10, "Marian"};
    void* p = static_cast<void*>(mc);
    MyClass* mc2 = static_cast<MyClass*>(p);
    std::cout << mc2->name << std::endl; // "Marian"
    delete(mc);

    // use operator new to allocate untyped memory block
    void* pvoid = operator new(1000);
    char* pchar = static_cast<char*>(pvoid);
    for(char* c = pchar; c < pchar + 1000; ++c)
    {
        *c = 0x00;
    }
    func(pvoid, 1000);
    char ch = static_cast<char*>(pvoid)[0];
    std::cout << ch << std::endl; // 'A'
    operator delete(pvoid);
}

Pointers to functions

In C-style programming, function pointers are used primarily to pass functions to other functions. This technique allows the caller to customize the behavior of a function without modifying it. In modern C++, lambda expressions provide the same capability with greater type safety and other advantages.

A function pointer declaration specifies the signature that the pointed-to function must have:

// Declare pointer to any function that...

// ...accepts a string and returns a string
string (*g)(string a);

// has no return value and no parameters
void (*x)();

// ...returns an int and takes three parameters
// of the specified types
int (*i)(int i, string s, double d);

The following example shows a function combine that takes as a parameter any function that accepts a std::string and returns a std::string. Depending on the function that's passed to combine, it either prepends or appends a string.

#include <iostream>
#include <string>

using namespace std;

string base {"hello world"};

string append(string s)
{
    return base.append(" ").append(s);
}

string prepend(string s)
{
    return s.append(" ").append(base);
}

string combine(string s, string(*g)(string a))
{
    return (*g)(s);
}

int main()
{
    cout << combine("from MSVC", append) << "\n";
    cout << combine("Good morning and", prepend) << "\n";
}

Raw pointers (C++)

Initialization and member access

Pointer arithmetic and arrays

`void*` pointers

Pointers to functions

See also

Feedback

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Additional resources

Raw pointers (C++)

Initialization and member access

Pointer arithmetic and arrays

void* pointers

Pointers to functions

See also

Feedback

Feedback

Additional resources

`void*` pointers